Fix GCC 4.2.1 to honor --sysroot for includes.
- Change the C++ directory entries to honor --sysroot if it is set.
- Don't define CROSS_INCLUDE_DIR for the cross compiler. Instead, set TARGET_SYSTEM_ROOT to point to WORLDTMP and always define STANDARD_INCLUDE_DIR.
- Change STANDARD_INCLUDE_DIR and the C++ include directories to just start with "/usr" always. The compiler will prepend the sysroot when doing cross-builds. GCC_INCLUDE_DIR (which contains headers that ship with the compiler such as intrinsincs rather than OS-supplied headers) remains hardcoded to look in TOOLS_PREFIX.
Reviewed by: bdrewery (older version)
Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D15127